Method for recovering rare earth elements

Abstract

The purpose of the present invention is to provide a method for recovering rare earth elements, with which it is possible to roughly separate a certain rare earth element from the other rare earth elements among the rare earth elements while recovering the rare earth elements from blast furnace slag. This method for recovering rare earth elements comprises: a leaching step in which blast furnace slag is brought into contact with an acid so as to leach the blast furnace slag, thereby obtaining a leachate 1 that contains the rare earth elements; a precipitation step in which an alkali is added to the leachate 1 so as to adjust the pH to a predetermined value, thereby obtaining a leachate 2 that contains a precipitate containing the rare earth elements; and a solid-liquid separation step in which the precipitate is separated from the leachate 2.

Description

Method for recovering rare earth elements

[0001] The present invention relates to a method for recovering rare earth elements, and more particularly, to a method for recovering rare earth elements that can roughly separate certain rare earth elements from other rare earth elements while recovering rare earth elements from blast furnace slag.

[0002] Rare earth elements are also called REE (Rare Earth Element), and are a general term for a total of 17 elements including Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Rare earth elements are used as additives for various materials such as hydrogen storage alloys, secondary battery raw materials, optical glass, rare earth magnets, phosphors, abrasives, and aluminum, and are a group of elements with high industrial value. However, the ore-bearing areas of rare earth elements are unevenly distributed and have a high scarcity value, so the supply volume is globally small, and there is also a problem of drastic price fluctuations due to changes in the social situation. Therefore, establishing a method for stably and abundantly supplying rare earth elements is important for the development of the industry.

[0003] As prior art, there are inventions of REE recovery methods targeting tin slag, fly ash, Ni ore, bauxite residue, etc.

[0004] Patent Document 1 discloses, for the purpose of efficiently recovering high-grade scandium from nickel oxide ore, solid-liquid separation of nickel oxide ore into a leachate and a leach residue under high temperature and high pressure together with sulfuric acid, etc.

[0005] Patent Document 2 aims to separate both precious metals and rare earth elements contained in fly ash, and discloses dissolving precious metals in a first extraction process and dissolving and separating the rare earth elements in a second extraction process, etc.

[0006] Patent Document 3 aims to provide a rare earth metal recovery method that can easily and surely selectively recover rare earth metals from tin slag containing rare earth metals, etc., and discloses dissolving tin slag with an inorganic acid, adding an oxidizing agent and a neutralizing agent to this solution so that radioactive substances precipitate and rare earth metals and iron do not precipitate, etc.

[0007] Japanese Patent Publication No. 5652503, Japanese Patent Publication No. 6159731, Japanese Patent Publication No. 5825074, Japanese Unexamined Patent Publication No. 2018-530673, Japanese Unexamined Patent Publication No. 2022-110887, International Publication No. 2023 / 032202

[0008] Incidentally, steel production accounts for the majority of total metal production, and the amount of steel slag produced as a by-product is also very large. For example, in Japan, more than 100 million tons of crude steel are produced annually, and as by-products, approximately 23 million tons of blast furnace slag, approximately 12 million tons of steelmaking slag, and approximately 3 million tons of electric furnace slag are produced annually. The iron ore, coal, limestone, and iron scrap that are the raw materials for these steel slags contain trace amounts of rare earth elements, and after the steelmaking process, trace amounts of rare earth elements are also contained in the steel slag. In particular, blast furnace slag has a higher concentration of rare earth elements than steelmaking slag and electric furnace slag. If rare earth elements can be recovered from blast furnace slag, which is produced in large quantities as a by-product and has a relatively high concentration of rare earth elements, it would be desirable to recover a large amount of rare earth elements.

[0009] Blast furnace slag contains trace amounts of rare earth elements, but its main components are silicon (Si), iron (Fe), aluminum (Al), magnesium (Mg), calcium (Ca), etc. A process is needed to separate these main components from the rare earth elements and recover the rare earth elements.

[0010] In addition, rare earth elements consist of 17 different elements, and their separation is required depending on the final application. While separation and purification are possible by utilizing the differences in the chemical properties of each element, streamlining this separation and purification process is extremely important from the perspective of industrial production management, including cost reduction.

[0011] In particular, blast furnace slag contains scandium (Sc), which has high industrial value, and it is desirable to efficiently separate and recover it.

[0012] Patent documents 1 to 3 relate to the separation of rare earth elements, but the starting material is not blast furnace slag. The raw materials targeted by these prior arts differ from blast furnace slag in terms of their leaching characteristics to acid, as well as the types and concentrations of the main constituent elements to be separated. Therefore, it has been difficult to efficiently recover rare earth elements, especially Sc, from steel slag using these prior arts.

[0013] Furthermore, while Patent Document 4 aims to separate not only rare earth elements but also metal salts other than rare earth elements from steel slag, the specific process conditions are not optimal for that purpose. Specifically, in the leaching process from steel slag, "saline solution, chelating agent, acid (0.1 mol / L HCl or HNO)" is used. ３ Although only ) is used, steel slag is basic, and the degree of its basicity changes, so with a fixed acid concentration (0.1 mol / L), there is a problem that the leaching efficiency changes greatly depending on the ratio of steel slag to acid.

[0014] Patent Document 5 describes a method for recovering rare earth elements from steel slag, and discloses that in the leaching process, "the pH of the leachate obtained by contacting it with an acid is adjusted to a predetermined final pH" or "the amount of additional acid or base added is controlled while monitoring the pH of the leachate obtained by contacting it with an acid." However, while Patent Document 5 considers the separation of radioactive elements (typically thorium), it does not disclose the separation of rare earth elements.

[0015] Patent Document 6 relates to a method for recovering rare earth elements from steel slag, and discloses a method in which the pH of the second leaching step is lower than the pH of the first leaching step during the leaching process. This allows for the recovery of rare earth elements from steel slag while also performing coarse separation of the rare earth elements. The separation efficiency of rare earth elements, for example, the separation efficiency of Sc (Sc concentration in the final extract / total rare earth element concentration) is 0.58 to 0.72.

[0016] As described above, methods for industrially recovering rare earth elements from blast furnace slag are not yet well established, and more appropriate methods are needed. The present invention aims to provide a method for recovering rare earth elements that can recover rare earth elements from blast furnace slag while also roughly separating certain rare earth elements from other rare earth elements.

[0017] The inventors of this invention conducted extensive research to solve the above-mentioned problems. Through this research, they obtained the following findings.

[0018] When rare earth elements in blast furnace slag were leached (dissolved) with acid to obtain a leachate, and then a basic solution was added to the leachate to adjust the pH to weakly acidic for operations such as solvent extraction, a gel-like precipitate (hereinafter referred to as precipitate) was formed due to Si and Al in the leachate. This is a phenomenon unique to when blast furnace slag is used as a raw material. One possible reason for this phenomenon is that when blast furnace slag is leached with acid, it is unavoidable that not only rare earth elements but also Si and Al will leach (dissolve).

[0019] This precipitate has the property of adsorbing REE, and furthermore, the adsorption behavior of rare earth elements differed depending on the pH of the leachate. It is generally known that there are differences in the solubility of elements, and that this also depends on pH. However, SiO ２ Ya Al ２ O ３ It had not been revealed that controlling the pH of a blast furnace slag extract, which is mainly composed of [unclear], to a predetermined level can result in differences in the adsorption rate of rare earth elements onto the precipitate.

[0020] The inventors of this invention focused on the fact that precipitates are formed from the leachate of blast furnace slag, and that the adsorption behavior of rare earth elements to the precipitates differs depending on the pH of the leachate, and thus completed the present invention.

[0021] In other words, the present invention includes the following embodiments: (1) A method for recovering rare earth elements from blast furnace slag, comprising: a leaching step of contacting the blast furnace slag with an acid to leach it and obtain a leachate 1 containing the rare earth elements; a precipitation step of adding an alkali to the leachate 1 to adjust it to a predetermined pH and obtain a leachate 2 containing precipitates containing the rare earth elements; and a solid-liquid separation step of separating the precipitates from the leachate 2. (2) The method for recovering rare earth elements according to (1), characterized in that in the leaching step, the pH of the leachate 1 is adjusted to less than 3.5. (3) The method for recovering rare earth elements according to (1) or (2), characterized in that in the precipitation step, the pH of the leachate 2 is adjusted to 3.5 or higher. (4) A method for recovering rare earth elements according to any one of (1) to (3), characterized in that the pH of the leachate 2 is adjusted to 3.5 or more and less than 5.0 in the precipitation step. (5) A method for recovering rare earth elements according to (1) or (2), characterized in that the pH of the leachate 2 is adjusted to 7.0 or more in the precipitation step.

[0022] The present invention provides a method for recovering rare earth elements from blast furnace slag, while simultaneously performing coarse separation of certain rare earth elements from other rare earth elements. By controlling the pH during the precipitation process, it is possible to control the adsorption of different rare earth elements onto the precipitate, thereby enabling coarse separation of rare earth elements. Typically, it is possible to separate almost exclusively Sc from the rare earth elements.

[0023] By separating and adsorbing rare earth elements, it is possible to concentrate and recover those rare earth elements using solvent extraction or ion exchange methods, which are commonly used to recover rare earth elements. Typically, it is possible to concentrate and recover only Sc using a precipitate that has adsorbed almost exclusively Sc.

[0024] Figure 1 is a flowchart illustrating an example of a method for recovering rare earth elements. Figure 2 is a photograph recording the formation of precipitates when the pH is adjusted in the range of 3.0 to 10.4. Figure 3 is a diagram showing the relationship between the distribution rate of each element in the precipitate and the pH of the leachate 2 adjusted in the precipitation process.

[0025] The following describes specific embodiments of the present invention (hereinafter referred to as "these embodiments") in detail with reference to the drawings. However, the present invention is not limited in any way to the following embodiments, and can be implemented with appropriate modifications without altering the gist of the present invention.

[0026] Although there are multiple definitions of elements included in the term "rare earth elements," the definition of rare earth elements presented in this disclosure refers to a group of 17 elements that include lanthanoids (Ln), consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), plus yttrium (Y) and scandium (Sc).

[0027] The blast furnace slag described herein is defined as blast furnace slag generated in the ironmaking process.

[0028] Figure 1 is a flowchart illustrating an exemplary method for recovering rare earth elements. The general outline of the rare earth element recovery method is as follows: Blast furnace slag is leached (dissolved) with an acid such as sulfuric acid, nitric acid, hydrochloric acid, or a mixture thereof to obtain a leachate [leaching step]; precipitates are obtained by adjusting the pH of the leachate [precipitation step] (adsorption of rare earth elements to the precipitates also occurs here); the precipitates are separated [solid-liquid separation step] to recover the rare earth elements.

[0029] The essential points of this invention are as follows: A leachate containing dissolved Si and Al, which are the main components of blast furnace slag, is obtained from blast furnace slag by acid leaching. By adjusting (increasing) the pH of the leachate, a gel-like precipitate consisting of Si and Al is formed. The leachate also contains rare earth elements dissolved from the slag, and these rare earth elements are adsorbed onto the precipitate. Furthermore, the adsorption behavior of each rare earth element changes depending on the pH. In other words, by adjusting the pH, rare earth elements can be selectively adsorbed onto the precipitate. The precipitate containing the adsorbed rare earth elements can be separated from the leachate using solid-liquid separation to recover the rare earth elements.

[0030] Typically, in the leaching process, blast furnace slag is brought into contact with acid to obtain leachate 1. Leachate 1 contains dissolved Si and Al, which are the main components of blast furnace slag, as well as dissolved rare earth elements.

[0031] In the precipitation process, alkali is added to the leachate obtained in the leaching process to adjust the pH. Because alkali is added, the pH increases. As the pH is adjusted and rises, a gel-like precipitate (hereinafter referred to as precipitate) consisting of Si and Al, etc., that was dissolved in the leaching process is generated. This leaching containing the precipitate is called leaching 2 and is distinguished from leaching 1 obtained in the aforementioned leaching process 1. Rare earth elements dissolved in leaching 1 are also adsorbed onto this precipitate. Here, the 17 types of rare earth elements each exhibit different adsorption behaviors. For example, at a certain pH, some rare earth elements may be adsorbed onto the precipitate, while others may not. Therefore, by adjusting the pH, rare earth elements can be selectively adsorbed onto the precipitate. It is also possible to adsorb all rare earth elements onto the precipitate.

[0032] In the solid-liquid separation step, the precipitate obtained in the precipitation step is separated from the leachate 2. Since rare earth elements can be selectively adsorbed onto the precipitate in the precipitation step, the selectively adsorbed rare earth elements can be separated and recovered.

[0033] Although not a mandatory feature, the liquid remaining after separating the precipitate from the leachate 2 contains dissolved rare earth elements that were not adsorbed by the precipitate. The rare earth elements can also be recovered by performing solid-phase extraction or solvent extraction on this liquid, followed by further separation operations such as precipitation, solid-liquid separation, or roasting.

[0034] By using the method of the present invention, rare earth elements can be leached from blast furnace slag. Therefore, rare earth elements can be recovered from blast furnace slag, even from raw materials containing many impurities such as blast furnace slag. Furthermore, by controlling the pH in the precipitation process, it is possible to control the adsorption of different rare earth elements onto the precipitate, thereby enabling coarse separation of rare earth elements. In other words, coarse separation of rare earth elements can be performed simultaneously. This reduces the burden of the refining process and allows for the recovery of rare earth elements at a lower cost. In particular, Sc, which has industrial value among rare earth elements, can be coarsely separated in the leaching process, which is the initial stage of the entire rare earth element recovery process.

[0035] Although not essential, the blast furnace slag may be crushed to increase its surface area prior to the leaching process. Alternatively, the molten blast furnace slag may be subjected to a water granulation treatment during cooling to produce fine-grained granulated slag with a large surface area. Increasing the surface area of ​​the blast furnace slag can increase the leaching rate of rare earth elements in the blast furnace slag.

[0036] <Leaching Process> In the leaching process, blast furnace slag is brought into contact with an acid to leach and dissolve the main components, such as Si and Al, and rare earth elements, from the blast furnace slag into the acidic solvent. More specifically, the blast furnace slag is brought into contact with an acid to leach it and dissolve the Si and Al, and rare earth elements contained in the blast furnace slag into the acidic solvent. A leachate 1 is obtained in which the blast furnace slag has been leached and Si and Al, and rare earth elements have been dissolved. The acid may be selected to leach and dissolve the blast furnace slag. If at least a portion of the blast furnace slag is leached, the effects of the present invention can be obtained with respect to the leached and dissolved leachate 1. The more blast furnace slag is leached with acid, the less slag residue remains, which reduces the effort required for subsequent separation of the slag residue, so this is preferable. In this respect, the entire amount of blast furnace slag may be leached and dissolved with acid.

[0037] This can be carried out by monitoring the pH of the leachate 1 obtained by contacting blast furnace slag with an acid and controlling the amount of additional acid added. From the viewpoint of sufficiently leaching or dissolving the blast furnace slag, a low pH of leachate 1 is preferable, and may be less than 3.5. Examples of acids include inorganic acids and organic acids, with inorganic acids being preferred. Specific examples of inorganic acids include sulfuric acid, nitric acid, hydrochloric acid, or mixtures thereof.

[0038] The following is an example of a specific method for carrying out the leaching process. Blast furnace slag is charged into the leaching reaction vessel, and acid is added. At this time, the solid-liquid ratio of blast furnace slag to acid is preferably 1:1 to 1:100. The pH of the added acid may be 1.0 or less. Since blast furnace slag contains 10 to 70% oxides and hydroxides of silicon, aluminum, calcium, magnesium, etc., the amount of acid to be initially added and its pH should be determined with the expectation that these will neutralize the slag. One method for monitoring the pH is to immerse a pH sensor in the leaching solution 1 obtained by contacting the blast furnace slag with acid and monitor the pH of the leaching solution 1 as it occurs. It is also preferable to stir the solution as needed to ensure that the pH of the leaching solution 1 becomes uniform. Note that adjusting the pH to a target constant value means that the acid or base is added so that the pH does not deviate by ±0.5, preferably ±0.3, and more preferably ±0.1 or more from the target pH for at least one minute continuously. Furthermore, while rare earth elements can be sufficiently leached even under conditions where the temperature of the leachate 1 is at room temperature (15-35°C) and the pressure is at atmospheric pressure (approximately 1013 hPa), the rare earth elements may be leached while heating or pressurizing to increase the leaching rate.

[0039] If the pH of leachate 1 is higher than the target pH, the pH can be adjusted to the target pH by adding small amounts of acid while monitoring the pH value. On the other hand, if the pH of leachate 1 is lower than the target pH, the pH can be adjusted to the target pH by adding small amounts of blast furnace slag while monitoring the pH value.

[0040] The reference pH can be appropriately determined according to the degree of leaching or dissolution of blast furnace slag. From the perspective of sufficiently leaching or dissolving blast furnace slag, it is preferable that the pH of the leachate 1 is lower, and it may be less than 3.5. The lower limit value of the pH may be selected from 0.0, 1.0, 1.5, 2.0, 2.5, or 3.0.

[0041] <Precipitation step> In the precipitation step, an alkali is added to the leachate Ⅰ obtained in the leaching step to adjust the pH. Since an alkali is added, the pH rises. When the pH is adjusted and rises, a gel-like precipitate (hereinafter referred to as precipitate) composed of Si, Al, etc. dissolved in the leachate occurs. The leachate containing this precipitate is referred to as leachate 2 and is distinguished from the leachate 1 obtained in the above-mentioned leaching step 1. The alkali (basic substance) is not particularly limited, and examples thereof include sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, or a mixed base thereof.

[0042] Generally, precipitates mainly composed of Si and Al begin to form when the pH is about 3.5 or higher. Also, as the pH increases, the amount of precipitate increases. Therefore, from the perspective of sufficiently obtaining (precipitating) the precipitate, the pH may be 3.5 or higher. Preferably, the lower limit value of the pH here may be selected from 4.0, 4.5, 5.0, 5.5, or 6.0.

[0043] In addition, rare earth elements dissolved in the leachate 1 are also adsorbed by this precipitate. That is, by setting the pH of the leachate 2 to about 3.5 or higher, a precipitate adsorbed with rare earth elements can be obtained. This precipitate can be separated and recovered from the leachate 2 by filtration or the like, and the rare earth elements contained in the precipitate can be recovered.

[0044] Here, the 17 rare earth elements each exhibit different adsorption behaviors towards the precipitate. For example, at a certain pH, one rare earth element may adsorb onto the precipitate, while another rare earth element may not. Therefore, by adjusting the pH, rare earth elements can be selectively adsorbed onto the precipitate. Generally, as the pH increases, rare earth elements are more likely to adsorb onto the precipitate. Thus, by increasing the pH, almost all of the rare earth elements can also be adsorbed onto the precipitate.

[0045] When the pH of the leachate 2 is in the range of 3.5 or more and less than 5.0, Sc among the rare earth elements is likely to adsorb onto the precipitate, while rare earth elements other than Sc are less likely to adsorb. Therefore, by adjusting the pH to 3.5 or more and less than 5.0, almost only Sc among the rare earth elements can be adsorbed onto the precipitate.

[0046] Generally, as the pH increases, the adsorption amount of Sc increases. Therefore, the pH may be adjusted according to the desired degree of Sc adsorption. Here, the lower limit value of the pH may be selected from 4.0, 4.5, and 5.0. On the other hand, as the pH increases, although slightly, the adsorption amount of rare earth elements other than Sc increases. Therefore, the upper limit value of the pH may be adjusted so that only Sc is adsorbed and rare earth elements other than Sc are not adsorbed. Here, the upper limit value of the pH may be selected from 5.0, 4.5, and 4.0.

[0047] When the pH of the leachate 2 is in the range of 5.0 or more, the adsorption amount of rare earth elements other than Sc increases. Therefore, the pH of the leachate 2 may be set to 5.0 or more. Generally, as the pH increases, the adsorption amount of rare earth elements other than Sc increases. When the pH of the leachate 2 is in the range of 7.0 or more, all of the rare earth elements including Sc are adsorbed onto the precipitate. Therefore, the pH may be adjusted according to the desired degree of adsorption. Here, the lower limit value of the pH may be selected from 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0, or 10.5. On the other hand, even if the pH is increased, the degree of adsorption may reach saturation. Therefore, the upper limit value of the pH may be adjusted. Here, the upper limit value of the pH may be selected from 10.5, 10.0, 9.5, 9.0, 8.5, 8.0, 7.5, or 7.0.

[0048] From the viewpoint of separating Sc from other rare earth elements, the following methods may be used. First, the pH of the leachate 2 is adjusted to a range of 3.5 or more and less than 5.0 so that almost only Sc is adsorbed onto the precipitate. By filtering the leachate 2 containing the precipitate, a precipitate containing almost only Sc among the rare earth elements can be recovered. In the liquid (leachate 2) after filtering the precipitate, rare earth elements other than Sc that are not adsorbed onto the precipitate are dissolved. By adding alkali to this liquid and adjusting the pH to 5.0 or higher, rare earth elements other than Sc can be precipitated, and further separation operations such as precipitation, solid-liquid separation, and roasting can be performed to recover the rare earth elements other than Sc.

[0049] The following is an example of a specific method for carrying out the precipitation process. The leachate 1 produced in the leaching process is charged into the precipitation reaction vessel (or the aforementioned leaching reaction vessel may be used with the leachate 1 still inside), and alkali (basic substance) is added. At this time, the liquid-to-liquid ratio of leachate 1 to alkali is preferably 1:1 to 100:1. The pH of the added alkali may be 10.0 or higher. Leachate 1 contains an acid for leaching and dissolving blast furnace slag, and the type and amount of alkali to be added initially should be determined with the expectation that it will be neutralized by this acid. As a method for monitoring the pH, one method is to immerse a pH sensor in leachate 2 obtained by adding alkali to leachate 1 and monitor the pH of leachate 2 as needed. It is also preferable to stir as appropriate to ensure that the pH of leachate 2 becomes uniform. Adjusting the pH to a target value means that an acid or base is added continuously for at least one minute so that the pH does not deviate by ±0.5, preferably ±0.3, and more preferably ±0.1 from the target pH. Furthermore, even under conditions where the leachate temperature is at room temperature (15-35°C) and the pressure is at atmospheric pressure (approximately 1013 hPa), the pH can be adjusted to generate precipitates. However, the pH may also be adjusted while heating or pressurizing to increase the precipitation rate.

[0050] If the pH of leachate 2 is lower than the target final pH, the pH can be adjusted to the target pH by adding small amounts of alkali while monitoring the pH value. The alkali (basic substance) to be added is not particularly limited, but examples include sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, or a mixture of these bases.

[0051] The pH of the leachate 2 can be appropriately determined depending on the purpose, and may be selected from a range consisting of any combination of upper limits of 10.5, 10.0, 9.5, 9.0, 8.5, 8.0, 7.5 or 7.0 and lower limits of 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, 10.0 or 10.5.

[0052] <Solid-Liquid Separation Process> The solid-liquid separation process is a process of separating the precipitate, which is the solid component, from the leachate 2 obtained in the precipitation process. In the solid-liquid separation process, the precipitate with adsorbed rare earth elements that appeared in the precipitation process can be physically separated from the liquid portion of the leachate. As for the separation method, known solid-liquid separation devices such as continuous thickeners, deep cone thickeners, lamellar thickeners, drum filters, disc filters, horizontal belt filters, filter presses, pressure filters, and centrifugal separators can be used.

[0053] The purity of the rare earth elements can be further increased by subjecting the precipitate containing adsorbed rare earth elements obtained through the leaching, precipitation, and solid-liquid separation processes, and the solution from which the precipitate has been separated from the leaching solution 2, to an optional purification process. Examples of optional purification processes include precipitation, secondary solid-liquid separation, and roasting. Illustrative methods for each process are described below. Note that the optional secondary solid-liquid separation process described here is referred to as a secondary solid-liquid separation process to distinguish it from the solid-liquid separation process that separates the precipitate from the leaching solution 2 mentioned above.

[0054] Furthermore, the liquid from which the precipitate has been separated from the leachate 2 may have its pH adjusted by adding alkali in the same manner as in the precipitation step described above. By increasing the pH, rare earth elements contained in the liquid can be precipitated. The precipitated rare earth elements can be physically separated from the liquid portion as precipitates in the same manner as in the solid-liquid separation step described above. Subsequently, the purity of the rare earth elements can be further increased by performing optional purification steps (such as precipitation, secondary solid-liquid separation, or roasting).

[0055] <Precipitation Process> The precipitation process is a process of adding a precipitating agent to a liquid containing rare earth elements to obtain a precipitate containing rare earth elements (hereinafter sometimes referred to as rare earth element precipitate). The liquid containing rare earth elements is, for example, a liquid obtained by adding a solvent to the precipitate separated from the leachate 2 after the solid-liquid separation process, the liquid after separating the precipitate from the leachate 2, or a liquid obtained by adding a solvent to the precipitate obtained by adjusting the pH of the liquid after separating the precipitate from the leachate 2, and may hereinafter be referred to as rare earth element-containing liquid. Examples of precipitating agents include bases and acids. Examples of bases include metal-containing basic salts such as sodium hydroxide and potassium hydroxide, and organic bases such as tetramethylammonium hydroxide (TMAOH), or mixed bases thereof, but organic bases are preferred from the viewpoint of obtaining rare earth elements of higher purity. Examples of acids include tartaric acid, carbonic acid, oxalic acid, or mixed acids thereof, and oxalic acid is preferred.

[0056] In the aforementioned precipitation process, using oxalic acid as the precipitant has the advantage of efficiently separating the rare earth elements from impurities such as aluminum and iron contained in the rare earth element-containing liquid (rare earth element-containing liquid), but it has the disadvantage of low separation efficiency from magnesium and calcium. On the other hand, using a base as the precipitant has the advantage of efficiently separating the rare earth elements from impurities such as magnesium and calcium contained in the rare earth element-containing liquid (rare earth element-containing liquid), but it has the disadvantage of low separation efficiency from iron and aluminum. The impurities contained in the rare earth element-containing liquid (rare earth element-containing liquid) vary depending on the composition of the blast furnace slag, and the problematic impurity elements differ depending on the intended use of the rare earth elements, so the precipitant should be selected according to the purpose.

[0057] <Secondary Solid-Liquid Separation Process> In the secondary solid-liquid separation process, the rare earth element precipitate obtained in the precipitation process and the resulting solution can be physically separated. As for the separation method, known solid-liquid separation devices such as continuous thickeners, deep cone thickeners, lamellar thickeners, drum filters, disc filters, horizontal belt filters, filter presses, pressure filters, and centrifugal separators can be used.

[0058] <Roasting Process> The roasting process is a process of roasting the rare earth element precipitate separated in the secondary solid-liquid separation process to obtain rare earth element oxides. The roasting process may include a washing process and a heating process. In the washing process, for example, the rare earth element precipitate separated in the secondary solid-liquid separation process may be washed with water to remove impurities. In the heating process, the rare earth element precipitate that has undergone the washing process may be heated to remove water, and volatile elements such as carbon, phosphorus, and nitrogen may be vaporized and removed, and reacted with oxygen to obtain rare earth element oxides.

[0059] In the roasting process described above, the roasting conditions are not limited, but for example, heating in a tubular furnace at approximately 900°C for about two hours is sufficient. Alternatively, by using a continuous furnace such as a rotary kiln, drying and roasting can be performed in the same apparatus, enabling the industrially efficient production of rare earth element oxides.

[0060] Although not shown in Figure 1, optional purification steps may be performed to further increase the purity of the rare earth elements. Optional purification steps include pH adjustment and extraction steps (including solid-phase extraction and / or solvent extraction).

[0061] <pH Adjustment Step> Although not essential, the pH of the liquid containing the rare earth elements (rare earth element-containing liquid) may be adjusted by adding a base or acid. Each element contained in the liquid has different precipitation or precipitation behavior. For example, at a certain pH, some elements may precipitate or precipitate, while others may not. By appropriately adjusting the pH of the liquid, the desired elements can be precipitated or precipitated due to the differences in the precipitation or precipitation behavior of each element. In a typical example, it is possible to separate the main components of blast furnace slag, such as silicon, aluminum, calcium, and magnesium, from the rare earth elements. <Extraction Step> The extraction step is a step of extracting the rare earth elements from the liquid containing the rare earth elements (rare earth element-containing liquid) to obtain a rare earth element concentrate. The extraction step preferably includes one or both of the following: a solid-phase extraction step that concentrates the rare earth elements by the principle of solid-phase extraction, and a solvent extraction step that concentrates the rare earth elements by the principle of solvent extraction. When using both the solid-phase extraction process and the solvent extraction process, the order in which the liquids are processed does not matter. When using only one of the extraction processes, the equipment can be simplified and rare earth elements can be recovered at a low cost, but the purity of the obtained rare earth elements will be lower compared to when both the solid-phase extraction process and the solvent extraction process are used. On the other hand, when rare earth elements are recovered using both the solid-phase extraction process and the solvent extraction process, the rare earth elements can be recovered with high purity, but the equipment will be larger and the equipment cost will be higher compared to when only one of the two processes is used. The advantages and disadvantages of these processes should be considered according to the purpose. For example, when using rare earth elements in applications where the inclusion of some impurities is not a problem, such as in mischmetal, one extraction process is sufficient. On the other hand, for applications where the purity of the product is particularly important, such as in the electronics industry, an extraction process combining the solid-phase extraction process and the solvent extraction process is desirable.

[0062] <Solid-phase extraction process> The solid-phase extraction process is a process included in the extraction process, and for example, it includes a rare-earth element adsorption process, a cation removal process, and a rare-earth element elution process, respectively. If necessary, a solid-phase washing process can be carried out in each of the solid-phase extraction processes. In the rare-earth element adsorption process, for example, a solid-phase extractant such as a solid phase made of a resin having iminodiacetic acid as a functional group may be brought into contact with the leachate to adsorb cations onto the solid phase and obtain a cation-adsorbed solid phase. Subsequently, in the cation removal process, for example, an inorganic acid of less than 0.3 N may be brought into contact with the cation-adsorbed solid phase to elute cations other than rare-earth elements from the solid phase and obtain a rare-earth element adsorbed solid phase. In the rare-earth element elution process, for example, an inorganic acid of 0.3 N or more and less than 3 N may be brought into contact with the rare-earth element adsorbed solid phase to elute the rare-earth elements and obtain a rare-earth element solid phase eluent. In the solid-phase washing step, for example, the solid phase that has undergone the rare-earth element elution step may be brought into contact with an inorganic acid of 3N or higher to elute impurity cations.

[0063] Although not essential, the solid phases that have undergone the solid phase washing process can be reused as solid phases in the aforementioned rare earth element adsorption process. Repeated use of the solid phase reduces the consumption of resin with iminodiacetic acid as a functional group, which is economically advantageous. However, if the same solid phase is used repeatedly, the resin with iminodiacetic acid as a functional group will deteriorate, reducing its performance and decreasing its ability to separate impurities and rare earth elements. In this case, replacing it with a new resin with iminodiacetic acid as a functional group will restore the performance to its original state.

[0064] <Solvent Extraction Step> The solvent extraction step is a step included in the extraction step, and for example, it is a step that includes a solvent extraction step and a reverse solvent extraction step. In the solvent extraction step, for example, an organic solvent containing a solvent extractant may be mixed with a liquid containing rare earth elements (rare earth element-containing liquid) to partition the rare earth elements into the organic solvent to obtain a rare earth element-containing organic phase. In the reverse solvent extraction step, for example, the rare earth element-containing organic phase obtained in the solvent extraction step may be mixed with water to partition the rare earth elements from the organic phase to the aqueous phase to perform reverse solvent extraction. In the reverse solvent extraction step, the pH may be adjusted as needed. The pH may be set to a pH suitable for the organic solvent used, etc.

[0065] In the solvent extraction step and the reverse solvent extraction step, when mixing the organic phase and the aqueous phase, known solvent extraction devices such as centrifugal extractors and pulsed columns may be used.

[0066] The solvent extractants used in the solvent extraction process can be conventionally known extractants, such as carboxylic acid-based extractants like neodecanoic acid, organophosphate-based extractants like di(2-ethylhexyl)phosphate, tributyl phosphate, and trioctylphosphine oxide, and amine-based extractants like triisooctylamine. The solvent extractant can be used without a solvent, or it can be dissolved in an organic solvent that does not mix with water, such as kerosene, xylene, or toluene. The solvent extractant may also be added to the aqueous phase.

[0067] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these descriptions.

[0068] (Example 1) Prior to the leaching process, blast furnace slag with a particle size of several mm to several cm was crushed in a ball mill to increase its surface area. The elemental composition of the blast furnace slag used in the experiment, determined by X-ray fluorescence analysis, is shown in Table 1 as an example.

[0069] Next, in the leaching process, the blast furnace slag was brought into contact with an acid (1 mol / L hydrochloric acid) with a solid-liquid weight ratio of 1:100 to dissolve the blast furnace slag. That is, the main components such as Si and Al, as well as rare earth elements, were leached from the blast furnace slag into the acidic solvent and dissolved. From the viewpoint of sufficiently leaching and dissolving the blast furnace slag, the pH of the leached solution 1 was monitored during leaching to maintain a pH of 3.0. If the pH was higher than the target value, acid was added, and if the pH was lower than the target value, blast furnace slag was added to maintain the target pH.

[0070] As a precipitation step, ammonia water was added as an alkali to leachate 1 obtained in the leaching step, and the pH was adjusted to a range of 3.0 to 10.4. Figure 2 is a photograph recording the formation of precipitates when the pH was adjusted to a range of 3.0 to 10.4. The formation of gel-like precipitates was confirmed at a pH of 3.5 or higher.

[0071] As a solid-liquid separation step, the leachate 2 obtained in the precipitation step was filtered to recover the precipitate.

[0072] We investigated the extent to which each element contained in the blast furnace slag (such as Si and Al, which are the main components of the slag, as well as rare earth elements, etc.) was distributed to the precipitate. The distribution ratio of each element to the precipitate is calculated using the following formula (1).

[0073]

[0074] In equation (1), the concentration before pH adjustment refers to the concentration of each element in leachate 1 obtained in the leaching process, and was subjected to elemental analysis using an inductively coupled plasma mass spectrometer. The volume of solution before pH adjustment is the volume of leachate 1. The concentration after pH adjustment refers to the concentration of each element in the liquid remaining after separating the precipitate from leachate 2 in the solid-liquid separation process, and was subjected to elemental analysis using an inductively coupled plasma mass spectrometer. The volume of solution after pH adjustment is the volume of solution remaining after separating the precipitate from leachate 2 in the solid-liquid separation process.

[0075] Figure 3 shows the relationship between the distribution rate of each element in the precipitate and the pH of the leachate 2 adjusted in the precipitation process. From Figure 3, it was found that the precipitate is mainly formed of Si and Al, that Sc is adsorbed into the precipitate at a lower pH than other rare earth elements, that almost only Sc is adsorbed into the precipitate at pH 3.5 to 5.0, and that rare earth elements other than Sc are also adsorbed into the precipitate at pH 5.0 to 7.0.

[0076] For example, by adjusting the pH of leachate 2 to 3.5-5.0 in the precipitation process and separating and recovering the precipitate in the solid-liquid separation process, it is possible to separate Sc from other rare earth elements. More specifically, when the pH of leachate 2 was adjusted to 4, it was possible to recover a precipitate containing 92% by mass of Sc in the blast furnace slag, but without other rare earth elements.

Claims

1. A method for recovering rare earth elements from blast furnace slag, comprising: a leaching step of contacting the blast furnace slag with an acid to leach it and obtain a leachate 1 containing the rare earth elements; a precipitation step of adding an alkali to the leachate 1 to adjust it to a predetermined pH and obtain a leachate 2 containing precipitates containing the rare earth elements; and a solid-liquid separation step of separating the precipitates from the leachate 2.

2. The method for recovering rare earth elements according to claim 1, characterized in that, in the leaching step, the pH of the leaching solution 1 is adjusted to less than 3.

5.

3. The method for recovering rare earth elements according to claim 1 or 2, characterized in that the pH of the leachate 2 is adjusted to 3.5 or higher in the precipitation step.

4. The method for recovering rare earth elements according to claim 1 or 2, characterized in that, in the precipitation step, the pH of the leachate 2 is adjusted to 3.5 or more and less than 5.

0.

5. A method for recovering rare earth elements according to claim 1 or 2, characterized in that the pH of the leachate 2 is adjusted to 7.0 or higher in the precipitation step.

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